24,809 research outputs found
Whirling Hexagons and Defect Chaos in Hexagonal Non-Boussinesq Convection
We study hexagon patterns in non-Boussinesq convection of a thin rotating
layer of water. For realistic parameters and boundary conditions we identify
various linear instabilities of the pattern. We focus on the dynamics arising
from an oscillatory side-band instability that leads to a spatially disordered
chaotic state characterized by oscillating (whirling) hexagons. Using
triangulation we obtain the distribution functions for the number of pentagonal
and heptagonal convection cells. In contrast to the results found for defect
chaos in the complex Ginzburg-Landau equation and in inclined-layer convection,
the distribution functions can show deviations from a squared Poisson
distribution that suggest non-trivial correlations between the defects.Comment: 4 mpg-movies are available at
http://www.esam.northwestern.edu/~riecke/lit/lit.html submitted to New J.
Physic
Hysteretic and chaotic dynamics of viscous drops in creeping flows with rotation
It has been shown in our previous publication
(Blawzdziewicz,Cristini,Loewenberg,2003) that high-viscosity drops in two
dimensional linear creeping flows with a nonzero vorticity component may have
two stable stationary states. One state corresponds to a nearly spherical,
compact drop stabilized primarily by rotation, and the other to an elongated
drop stabilized primarily by capillary forces. Here we explore consequences of
the drop bistability for the dynamics of highly viscous drops. Using both
boundary-integral simulations and small-deformation theory we show that a
quasi-static change of the flow vorticity gives rise to a hysteretic response
of the drop shape, with rapid changes between the compact and elongated
solutions at critical values of the vorticity. In flows with sinusoidal
temporal variation of the vorticity we find chaotic drop dynamics in response
to the periodic forcing. A cascade of period-doubling bifurcations is found to
be directly responsible for the transition to chaos. In random flows we obtain
a bimodal drop-length distribution. Some analogies with the dynamics of
macromolecules and vesicles are pointed out.Comment: 22 pages, 13 figures. submitted to Journal of Fluid Mechanic
Convergence of invariant densities in the small-noise limit
This paper presents a systematic numerical study of the effects of noise on
the invariant probability densities of dynamical systems with varying degrees
of hyperbolicity. It is found that the rate of convergence of invariant
densities in the small-noise limit is frequently governed by power laws. In
addition, a simple heuristic is proposed and found to correctly predict the
power law exponent in exponentially mixing systems. In systems which are not
exponentially mixing, the heuristic provides only an upper bound on the power
law exponent. As this numerical study requires the computation of invariant
densities across more than 2 decades of noise amplitudes, it also provides an
opportunity to discuss and compare standard numerical methods for computing
invariant probability densities.Comment: 27 pages, 19 figures, revised with minor correction
Thermoelectric enhancement in PbTe with K, Na co-doping from tuning the interaction of the light and heavy hole valence bands
The effect of K and K-Na substitution for Pb atoms in the rock salt lattice
of PbTe was investigated to test a hypothesis for development of resonant
states in the valence band that may enhance the thermoelectric power. We
combined high temperature Hall-effect, electrical conductivity and thermal
conductivity measurements to show that K-Na co-doping do not form resonance
states but2 can control the energy difference of the maxima of the two primary
valence sub-bands in PbTe. This leads to an enhanced interband interaction with
rising temperature and a significant rise in the thermoelectric figure of merit
of p-type PbTe. The experimental data can be explained by a combination of a
single and two-band model for the valence band of PbTe depending on hole
density that varies in the range of 1-15 x 10^19 cm^-3.Comment: 8 figure
Probing neutrino oscillations jointly in long and very long baseline experiments
We examine the prospects of making a joint analysis of neutrino oscillation
at two baselines with neutrino superbeams. Assuming narrow band superbeams and
a 100 kt water Cerenkov calorimeter, we calculate the event rates and
sensitivities to the matter effect, the signs of the neutrino mass differences,
the CP phase and the mixing angle \theta_{13}. Taking into account all possible
experimental errors under general consideration, we explored the optimum cases
of narrow band beam to measure the matter effect and the CP violation effect at
all baselines up to 3000 km. We then focus on two specific baselines, a long
baseline of 300 km and a very long baseline of 2100 km, and analyze their joint
capabilities. We found that the joint analysis can offer extra leverage to
resolve some of the ambiguities that are associated with the measurement at a
single baseline.Comment: 23 pages, 11 figure
The SPICE carbon isotope excursion in Siberia: a combined study of the upper Middle Cambrian-lowermost Ordovician Kulyumbe River section, northwestern Siberian Platform
An integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and
magnetostratigraphic study through the upper Middle Cambrianālowermost Ordovician shallowmarine
carbonates of the northwestern margin of the Siberian Platform is reported. The interval was
analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary
of the Enisej River. It comprises the upper Ustā-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower
Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here
in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the
excursion, showing Ī“13C positive values as high as+4.6ā°and least-altered 87Sr/86Sr ratios of 0.70909,
is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position
of the SPICE excursion does not support traditional correlation of the boundary between theOrakta and
Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia,
South China and Kazakhstan, where the Glyptagnostus stolidotus and G. reticulatus biozones are
known to immediately precede the SPICE excursion and span the MiddleāUpper Cambrian boundary.
The CambrianāOrdovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk
Formation, in which carbon isotope values show a local maximum below a decrease in the upper
part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity
sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian
times at 7ā10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic
intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian
on chemostratigraphic grounds contains only 10ā11 magnetic intervals
Non Mean-Field Quantum Critical Points from Holography
We construct a class of quantum critical points with non-mean-field critical
exponents via holography. Our approach is phenomenological. Beginning with the
D3/D5 system at nonzero density and magnetic field which has a chiral phase
transition, we simulate the addition of a third control parameter. We then
identify a line of quantum critical points in the phase diagram of this theory,
provided that the simulated control parameter has dimension less than two. This
line smoothly interpolates between a second-order transition with mean-field
exponents at zero magnetic field to a holographic
Berezinskii-Kosterlitz-Thouless transition at larger magnetic fields. The
critical exponents of these transitions only depend upon the parameters of an
emergent infrared theory. Moreover, the non-mean-field scaling is destroyed at
any nonzero temperature. We discuss how generic these transitions are.Comment: 15 pages, 7 figures, v2: Added reference
Signatures of Electronic Nematic Phase at Isotropic-Nematic Phase Transition
The electronic nematic phase occurs when the point-group symmetry of the
lattice structure is broken, due to electron-electron interactions. We study a
model for the nematic phase on a square lattice with emphasis on the phase
transition between isotropic and nematic phases within mean field theory. We
find the transition to be first order, with dramatic changes in the Fermi
surface topology accompanying the transition. Furthermore, we study the
conductivity tensor and Hall constant as probes of the nematic phase and its
transition. The relevance of our findings to Hall resistivity experiments in
the high- cuprates is discussed.Comment: 5 pages, 3 figure
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